1. Field of the Invention
The present invention relates generally to tracking control arrangements for use in optical disc players, and more particularly, is directed to an improved tracking servocontrol arrangement suitable for use in an optical disc player which can reproduce an information signal from a record track formed on an optical disc available for inscribing.
2. Description of the Prior Art
There has been proposed an optical disc available for inscribing of information which has generally concave preformed grooves on a recording surface thereof for providing address information of record tracks or the like. The optical disc for inscribing of a so-called land-type recording is provided on a recording surface thereof with a precut portion as shown in FIG. 1 of the accompanying drawings, for example.
In case of the optical disc shown in FIG. 1, a precut portion 2 is constituted with a bank 1 which is called a "land" and provided on a recording surface with a height corresponding to one eighth of the wavelength of a light beam impinging thereon and a plurality of grooves G which are provided on the bank 1 with a depth corresponding to one fourth of the wavelength of the light beam impinging on the bank 1 and by which address information signals or other information signals are recorded. A portion of the top surface of the bank 1 following the precut portion 2 forms a recording portion 3 available for inscribing information signals thereon.
In an optical disc player working with such an optical disc available for inscribing to record an information signal on a spiral record track composed of precut portions and recording portions formed on the optical disc as mentioned above and to reproduce an information signal recorded on the spiral record track formed on the optical disc, a light beam is used for inscribing the information signal on the recording portions in the spiral record track on the optical disc and for reading the information signal from the spiral record track on the optical disc. The light beam is directed to impinge on the optical disc caused to rotate through an optical head which is moved radially of the optical disc and is required to trace correctly the spiral record track on the optical disc. To make the light beam comply with this requirement, tracking servocontrol is performed for maintaining the light beam, which is directed onto the optical disc through the optical head moved radially of the optical disc, in correct tracking relation to the spiral record track on the optical disc in both the recording operation and the reproducing operation.
In order to carry out such tracking servocontrol, the optical disc player is provided with a tracking control arrangement which is operative to produce a tracking error signal representing positional deviation of a beam spot formed on the recording surface by the light beam from the center of each portion of the spiral record track, that is tracking error, in response to the output signal of a photosensing device to which the light beam directed through the optical head to the optical disc and then reflected from the recording surface of the optical disc is led, and to drive an optical element, for example, a lens or a mirror partially constituting the optical head, to control the position thereof in response to the tracking error signal.
A proposed optical system containing such a tracking control arrangement for use in the optical disc player is shown in FIG. 2. In FIG. 2, an optical disc D has a spiral record track composed of precut portions 2 and recording portions 3 such as shown in FIG. 1 on a recording surface thereof and is rotated at predetermined speed around the center thereof.
On the occasion of the reproducing operation, a laser light beam emitted from a laser light source 4 passes through a collimating lens 5 and enters beam splitters 6 and 7 aligned to pass through the same. The laser light beam, after having passed through the beam splitters 6 and 7, passes through a quarter-wave plate 8 and an object lens 9 which focusses the laser light beam on the recording surface of the disc D. The object lens 9 is set to be moved in a direction transverse to the record track on the bank 1 and in a direction perpendicular to the recording surface on the disc D by a tracking control driving device 10 and a focus control driving device 11, respectively. The laser light beam caused to impinge on the disc D is modulated in intensity in accordance with the record track thereon and then reflected therefrom to pass through the object lens 9 and the quarter-wave plate 8 into the beam splitter 7. A part of the laser light beam from the disc D is refracted to the right in FIG. 2 at the beam splitter 7 and the rest of the laser light beam from the disc D passes through the beam splitter 7 into the beam splitter 6 to be refracted thereat to the right in FIG. 2. The laser light beam refracted by the beam splitter 7 passes through a lens portion 12a to a photodetector 13 to be used for detection of tracking error of the laser light beam on the record surface of the disc D, while the laser light beam refracted by the beam splitter 6 passes through a lens portion 12b to the photodetector 13 to be used for detection of focus error of the laser light beam on the record surface of the disc D and for production of a reproduced information signal.
The photodetector 13 comprises a plurality of light detecting elements, each of which detects the laser light beam emerging from the beam splitter 7 or 6 and produces an output signal in response to the variations in intensity of the detected laser light beam. The output signals derived from the photodetector 13 are supplied to a signal processor 14 containing a tracking servocontrol circuit and a focus servocontrol circuit. The tracking servocontrol circuit produces a driving signal for tracking servocontrol which is supplied to the tracking control driving device 10 for moving the object lens 9 to control the position thereof so that the tracking servocontrol is performed, and the focus servocontrol circuit produces a driving signal for focus servocontrol for maintaining correct focusing of the laser light beam caused to impinge on the disc D, which is supplied to the focus control driving device 11 for moving the object lens 9 to control the position thereof so that the focus servocontrol is performed. Further, the reproduced information signal is also derived from the signal processor 14.
In the optical system described above, the tracking control arrangement is constituted with the light detecting elements forming the photodetector 13, the tracking servocontrol circuit in the signal processor 14 and the tracking control driving device 10, and one example of such tracking control arrangement proposed previously is shown in FIG. 3.
In the arrangement shown in FIG. 3, the photodetector 13 comprises a photodetecting device 15 including a pair of light detecting elements 15A and 15B for detecting tracking error and a photodetecting device 16 including a group of four light detecting elements 16A, 16B, 16C and 16D for detecting focus error and producing the reproduced information signal. The photodetecting devices 15 and 16 are so positioned that the laser light beam emerging from the beam splitter 7 forms a beam spot on the photodetecting device 15 and the laser light beam emerging from the beam splitter 6 forms a beam spot on the photodetecting device 16.
The light detecting elements 15A and 15B produce respectively output signals Sa and Sb. These output signals Sa and Sb are amplified by amplifires 17a and 17b, respectively, and then supplied to a subtracter 18 in which the difference between the output signals Sa and Sb is obtained. The subtracter 18 produces a tracking error signal TE, which corresponds to the difference between the output signals Sa and Sb and represents positional deviation of a spot formed on the recording surface of the disc D by the laser light beam from the center of each portion of the record track on the disc D and the tracking error signal TE is supplied to one of the input terminals of a dividing circuit 24. The amplitude of the tracking error signal TE varies in response to not only variations in the tracking error but also variations in the total quantity of light of the laser light beam received by photodetecting device 15.
At the same time, the light detecting elements 16A-16D produce respectively output signals Ra, Rb, Rc, and Rd. The output signals Ra and Rc derived respectively from the light detecting elements 16A and 16C are supplied to an adder 19 to be added to each other and an output signal Ra+Rc is obtained from the adder 19. Similarly, the output signals Rb and Rd derived respectively from the light detecting elements 16B and 16D are supplied to an adder 20 to be added to each other and an output signal Rb+Rd is obtained from the adder 20. Then, the output signals Ra+Rc and Rb+Rd derived respectively from the adders 19 and 20 are supplied to an adder 21 to be further added to each other and a reproduced information signal RF, which is identical to a signal Ra+Rb+Rc+Rd, is obtained from the adder 21. The output signals Ra+Rc and Rb+Rd are also supplied to a subtracter 22 in which the difference between the output signals Ra+Rc and Rb+Rd is obtained. The subtracter 22 produces a focus error signal FE which corresponds to the difference between the output signals Ra+Rc and Rb+Rd and represents defocusing of the laser light beam caused to impinge on the disc D.
The reproduced information signal RF obtained from the adder 21 has the respective different waveforms in the period in which the laser light beam caused to impinge on the recording surface of the disc D is tracing the precut portion 2 on the record track and in the period in which laser light beam caused to impinge on the recording surface of the disc D is tracing the recording portion 3 on the record track. By way of example, with a constant quantity of light of the laser light beam directed onto the disc D, during the period in which the laser light beam is tracing the precut portion 2, the reproduced information signal RF takes a high level which is laid between a level V.sub.1 and a level V.sub.2 higher than the level V.sub.1 and a low level which is lower than the level V.sub.1 so as to cross over the level V.sub.1 intermittently, as shown in FIG. 4. On the other hand, during the period in which the laser light beam is tracing the recording portion 3, the reproduced information signal RF takes a high level which is higher than the level V.sub.2 and a low level which is laid between the level V.sub.1 and the level V.sub.2 so as to cross over the level V.sub.2 intermittently when the laser light beam impinges on a recorded area R in the recording portion 3 where an information signal is recorded and also takes the low level between the level V.sub.1 and the level V.sub.2 continuously when the laser light beam impinges on a blank area N in the recording portion 3 where any information signal is not recorded yet. Further, the reproduced information signal RF obtained from the adder 21 has an amplitude variation with relatively low frequency which results from variations in the total quantity of light of the laser light beam received by the photodetecting device 16 and therefore is in proportion to the variations in the total quantity of light of the laser light beam received by the photodetecting device 15.
The reproduced information signal RF thus obtained is supplied to a low pass filter 23 and a control signal RFo, which has a level varying in response to the above mentioned amplitude variation with relatively low frequency of the reproduced information signal RF, is obtained from the low pass filter 23. This control signal RFo is supplied to the other of the input terminals of the dividing circuit 24 as a signal having an amplitude varying in proportion to the variations in the total quantity of light of the laser light beam received by the photodetecting device 15.
In the dividing circuit 24, the operation equivalent to a division for dividing a value corresponding to the amplitude of the tracking error signal TE by a value corresponding to the amplitude of the control signal RFo is performed, and a modified or normalized tracking error signal TE' which is intended to have an amplitude corresponding to the amplitude of the tracking error signal TE without the variation resulting from the variations in the total quantity of light of the laser light beam received by photodetecting device 15 and varying properly in response to the variations in the tracking error even though the total quantity of light of the laser light beam received by photodetecting device 15 is varied, is obtained from the dividing circuit 24. This modified tracking error signal TE' is supplied through a phase compensating circuit 25 and a driving circuit 26 to the tracking control driving device 10 as the driving signal for tracking servocontrol. The tracking control driving device 10 supplied with the driving signal for tracking servocontrol causes the laser light beam impinging on the recording surface of the disc D to move in the direction transverse to the record track on the recording surface to be in correct tracking relation to the record track.
When the tracking servocontrol is performed by the tracking control arrangement as described above in the reproducing operation in which the information signal recorded on the optical disc D having the spiral record disc composed of the precut portions 2 and the recording portions 3 as shown in FIG. 1 is read, since the average reflectivity at the recorded area R in the recording portions 3 is distinctively higher than the average reflectivity at the blank area N in the recording portion 3, the total quantity of light of the laser light beam received by the photodetecting devices 15 and 16 during the period in which the laser light beam impinges on the recorded area R is distinctively more than that during the period in which the laser light beam impinges on the blank area N. However, the amplitude of the tracking error signal TE' obtained from the dividing circuit 24 is not affected by the difference between the total quantity of light of the laser light beam received by the photodetecting device 15 during the period in which the laser light beam impinges on the recorded area R and the similar total quantity of light during the period in which the laser light beam impinges on the blank area N, and takes the same level both in the period in which the laser light beam impinges on the recorded area R and in the period in which the laser light beam impinges on the blank area N, if the degree of the tracking error is not varied between both periods.
Under such a condition, the amplitude of the tracking error signal TE is always modified in the dividing circuit 24 in response to the variations in the total quantity of light of the laser light beam received by the photodetecting device 15, so that the modified tracking error signal TE' is produced. Accordingly, the amplification of the tracking error signal TE in order to obtain the modified tracking error signal TE' from the dividing circuit 24 during the period in which the laser light beam impinges on the recorded area R is made to be smaller than that during the period in which the laser light beam impinges on the blank area N even if the degree of the tracking error is not varied between both periods, and consequently a loop gain with which the tracking servocontrol circuit performs the control operation is varied, so that the control operation is carried out with a relatively high gain when the laser light beam impinges on the blank area N and with a relatively low gain when the laser light beam impinges on the recorded area R. This results in the disadvantage that the tracking servocontrol is apt to be made unstable and the information signal may not be read properly from the spiral record track on the optical disc D.